When configuring the PCIe settings for "performance", we allow parentsto have a larger Max Payload Size than children and rely on childrenMax Read Request Size to not be larger than their own MPS to avoidhaving the host bridge generate responses they can't cope with.

However, various drivers in Linux call pci_set_readrq() with arbitraryvalues, assuming this to be a simple performance tweak. This breaksunder our "performance" configuration.

Fix that by making sure the value programmed by pcie_set_readrq() isnever larger than the configured MPS for that device.